Information signals, such as data signals, media signals and especially compressed video and audio streams propagate over various communication channels, such as but not limited to terrestrial, wireless, satellite, wired and cable communication channels. Media streams usually include large amounts of information.
The transmission of information signals usually involves frequency conversion and modulation. There are various types of modulation schemes including QPSK, QAM modulation and the like. QPSK modulation is more robust than QAM modulation but is less efficient. In some cases QAM modulation is used for downstream transmission towards end users whereas QPSK modulation is used for upstream transmission from the end users.
QAM modulators are characterized by the size of their constellation. For example, a 256 QAM modulator has a constellation of 256 symbols that is spread within a complex plane. Each symbol has an in-phase component as well as a quadrature component.
Typically, QAM modulation of information streams includes the stages of: (a) receiving baseband information streams, (b) optionally, processing the information stream (for example applying error correction algorithms) to provide processed streams, (c) converting groups of processed stream signals to symbols, (d) generating in-phase components and quadrature components of the symbols, and (e) performing quadrature modulation to provide a complex stream of radio frequency signals. In a 256 QAM modulator each symbol represents a group of eight consecutive bits, and each symbol has a four-bit quadrature component and a four-bit in-phase component.
The following U.S. patents and U.S. patent applications, which are incorporated herein by reference, provide an illustration of the state of the art QAM modulators: U.S. Pat. No. 6,049,572 of Hansen, titled “Optimization of QAM constellation space for auxiliary I-Q signaling”; U.S. patent application 20030206600 of Vankka, titled, “QAM modulator”; U.S. patent application 20030147472 of Bach et al., titled “High speed QPSK MMIC and QAM modulator”; U.S. Pat. No. 6,693,970 of Vankka, titled “QAM modulator”; U.S. Pat. No. 6,687,311 of Zhang, titled “Direct QAM modulator with digital feedback control and complex pre-equalization for phase and amplitude distortion”; U.S. Pat. Nos. 6,430,228 and 6,141,387 of Zhang, titled “Digital QAM modulator using post filtering carrier recombination”; U.S. Pat. Nos. 6,118,826 and 5,848,102 of Zehavi et al., titled “Method and apparatus for encoding/decoding QAM trellis coded data”; U.S. Pat. No. 5,852,389 of Kumar, titled “Direct QAM modulator”; U.S. Pat. No. 5,450,044 of Hulick, titled “Quadrature amplitude modulator including a digital amplitude modulator as a component thereof”; U.S. Pat. No. 5,153,536 of Muller, titled “Suppressed carrier modulator formed from two partial modulators each including a phase delay path”; and U.S. Pat. No. 4,999,590 of Verdot, titled “Four state phase shift modulator, in particular for amplitude modulation of two carriers in quadrature with a large number of states”.
FIG. 1 illustrates a prior art quadratue modulator 10. It converts an input in-phase baseband signal Ib(t) 12, and an input quadrature baseband signal Qb(t)14 to a single complex RF output signal RF(t)16.
Quadrature modulator 10 includes two mixers 22 and 24, local oscillator 30 and a 90° power splitter 36 and a combiner 40. The first mixer 22 receives as inputs Ib(t) 12 and a first carrier signal LOI(t) and outputs a modulated in-phase signal Im(t) 42. The second mixer 24 receives as inputs Qb(t) 14 and a second carrier signal LOQ(t) and outputs a modulated quadrature signal Qm(t) 44. The combiner 40 receives both modulated signals and outputs RF(t) 16.
Prior art quadrature modulators are characterized by gain, phase and offset imbalances between the I & Q branches. In addition to the modulated harmonics, these imbalances cause additional harmonics at the modulator output such as Carrier Leakage and Sideband.
FIG. 2 is a RF spectrum in which the x-axis represents the spectral components of the output signal of quadrature modulator 10 whereas the y-axis represents the amplitude or power of these spectral components. The spectral components include: (i) a carrier leakage component 52 at the carrier frequency (the frequency of the local oscillator 30); (ii) a desired output component 54 at a desired output frequency; and (iii) a sideband component 50 at a mirror frequency. These three spectral components are relatively proximate to each other thus merely performing spectral filtering is not effective.
Another model of a prior art quadrature modulator also includes two frequency dependent filters, one between each mixer and combiner, each represents frequency dependent effects that occur in the quadrature modulator.
The following U.S. patents and patent applications, as well as published articles, which are incorporated herein by reference, provide an illustration of the state of the art quadrature modulators and some also illustrate manners for quadrature modulation imbalance compensation: U.S. Pat. No. 6,618,096 of Stapleton, titled “System and method for adaptively balancing quadrature modulators for vestigial-sideband generation”; U.S. patent application 20040032913 of Dinur, titled “Method and apparatus of compensating imbalance of a modulator”; U.S. patent application 20040021516 of Oishi et al., titled “Distortion compensation apparatus”; U.S. patent application 20030231075 of Heiskala et al., titled “Amplitude imbalance compensation of quadrature modulator”; U.S. patent application 20030118121 of Makinen, titled “Method in digital quadrature modulator and demodulator, and digital quadrature modulator and demodulator”; U.S. patent application 20030095607 of Huang et al., titled “System and method for direct transmitter self-calibration”; U.S. patent application 20030053556 of Prabir, titled “Image-canceling quadrature modulator and method”; U.S. patent application 2002136324 of Nagasaka, titled “Circuit and method for compensating for non-linear distortion”; U.S. patent application 20030098752 and U.S. Pat. No. 6,657,510 of Haghighat, titled “Corrective phase quadrature modulator system and method”; U.S. patent application 20020071497 of Bengtsson et al., titled “IQ modulation systems and methods that use separate phase and amplitude signal paths and perform modulation within a phase locked loop”; U.S. patent application 20010016017 of Ishihara, titled “Quadrature modulator”; U.S. patent application 2004004515 of Takahashi, titled “Modulator and demodulator”; U.S. Pat. No. 6,700,453 of Heiskala et al., titled “Amplitude imbalance compensation of quadrature modulator”; U.S. Pat. No. 6,298,096 of Burgin, titled “Method and apparatus for determination of predistortion parameters for a quadrature modulator”; U.S. Pat. Nos. 6,208,698 and 5,883,551 of Marchesani et al, titled “Quadrature modulator imbalance estimator and modulator stage using it”; U.S. Pat. No. 5,847,619 of Kirisawa, titled “Method and system for calibrating a quadrature phase modulator”; U.S. Pat. No. 5,663,691 of Kowalik et al., titled “Estimator for estimating an operating defect in a quadrature modulator, and a modulation stage using the estimator”; U.S. Pat. No. 5,367,271 of Yamamoto, et al., titled “Quadrature modulator having phase shift and amplitude compensation circuits”; U.S. Pat. No. 5,705,958 of Janer, titled “Apparatus for correcting quadrature error in a modulator and/or in a demodulator for a signal having a plurality of phase states, a corresponding transmitter, and a corresponding receiver”; U.S. Pat. No. 5,293,406 of Suzuki, titled “Quadrature amplitude modulator with distortion compensation”; U.S. Pat. No. 5,105,195 of Conrad, titled “System and method for compensation of in-phase and quadrature phase and gain imbalance”; U.S. Pat. No. 4,890,301 of Hedberg, titled “Arrangement for compensating errors in a quadrature modulator”; U.S. patent application 20030231075 of Heiskala et al., titled “Amplitude imbalance compensation of quadrature modulator”; U.S. patent application 20040021516 of Oishi et al., titled “Distortion compensation apparatus”; U.S. patent application 20030045249 of Nielsen, titled “Feedback compensation detector for a direct conversion transmitter”; U.S. Pat. No. 5,355,101 of Ichihara et al., titled “Quadrature modulator having circuit for correcting phase error”; N. Vasudev and Oliver M. Collins, “Near-Ideal RF Upconverters” of N. Vasudev and Oliver M. Collins, published at IEEE Transactions on microwave theory and techniques, Volume 50, No. 11, November 2002; A. Guidi, P. McIllree and John Stannard, “Designing a high-speed modem for microwave, satellite communications”, www.rfdesign.com. November 2001; R. Cushing, “Single sideband up-conversion of quadrature DDS signals to the 800-to-2500 Mhz band”, Analog Dialogue, 34-3, 2000; J. Surber, C. Ventola, “Innovative Mixed-Signal Chipset Targets Hybrid-Fiber Coaxial Cable Modems”, Analog Dialogue, 31-3, 1997.
Quadrature receivers also have to cope with gain and phase imbalances. The following U.S. patent applications, which are incorporated herein by reference, provide an illustration of the state of the art quadrature receivers and some also refer to quadrature receivers modulation imbalance compensation: U.S. Pat. No. 6,490,326 of Bastani et el., titled “Method and apparatus to correct for in-phase and quadrature-phase gain imbalance in communication circuitry”; U.S. Pat. No. 637,902 of Walley, titled “Gain imbalance compensation method and apparatus for a quadrature receiver”; U.S. Pat. No. 5,396,565 of Jasper et al., titled “Method for determining desired components of quadrature modulated signals”; U.S. patent application 20030139167 of Ciccarelli et al., titled “System and method for I-Q mismatch compensation in a low IF or zero IF receiver”; U.S. patent application 20030206603 of Husted, titled “Systems and methods to provide wideband magnitude and phase imbalance calibration and compensation in quadrature receivers”; U.S. patent application 20020160741 of Kim et al., titled “Image rejection mixer with mismatch compensation”.
Since the sidebands and carrier leakage interfere with the desired modulated harmonics, there is a need to provide efficient methods for calibrating a quadrature modulator so as to remove these artifacts and to provide efficient modulation systems.